Work machines such as, for example, excavators, loaders, dozers, motor graders, and other types of heavy machinery use multiple actuators supplied with hydraulic fluid from a pump on the work machine to accomplish a variety of tasks. These actuators are typically velocity controlled based on an actuation position of an operator interface device. For example, an operator interface device such as a joystick, a pedal, or any other suitable operator interface device may be movable to generate a signal indicative of a desired velocity of an associated hydraulic actuator. When an operator moves the interface device, the operator expects the hydraulic actuator to move at an associated predetermined velocity. However, when multiple actuators are simultaneously operated, the hydraulic fluid flow from a single pump may be insufficient to move all of the actuators at their desired velocities. Situations also exist where the single pump is undersized and the desired velocity of a single actuator requires a fluid flow rate that exceeds the flow capacity of the single pump.
One method of selectively combining the hydraulic fluid flow from multiple pumps to move a single actuator is described in U.S. Pat. No. 4,345,436 (the '436 patent) issued to Johnson on Aug. 24, 1982. The '436 patent describes a hydraulic system having a first circuit supplied with fluid pressurized by a first pump, and a second circuit supplied with fluid pressurized by a second pump. Each of the first and second circuits have multiple fluid motors connected in series by way of bypass passages. In addition, one fluid motor of the first circuit is connected in series with the fluid motors of the second circuit, and one fluid motor of the second circuit is connected in series with the fluid motors of the first circuit. In this manner, if excess fluid exists within the first circuit, it is made available to the one fluid motor of the second circuit. Likewise, if excess fluid exists in the second circuit, it is made available to the one fluid motor of the first circuit. A group of resolver valves connects the highest pressure of the first circuit to the control of the first pump, and the highest pressure of the second circuit to the control of the second pump to thereby control the displacements and associated outputs of the first and second pumps. At times when fluid from one circuit is being delivered to the one motor of the other circuit, the pressure comparing function of the resolver group of the one circuit is extended to include the one motor of the other circuit.
Although the resolver group of the '436 patent may help control the output of the first and second pumps, even during flow sharing between the first and second circuits, it may be expensive, unreliable, and inefficient. In particular, the numerous resolver valves may increase the cost of the hydraulic system and reduce the reliability. In addition, because the first and second pumps are controlled in response to a pressure or flow fluctuation, rather than in anticipation of the fluctuation, the system may inherently include a time lag. This time lag could decrease the responsiveness and efficiency of the system. Further, it is possible for the resolver valves to induce sudden and extreme control changes in the first and second pumps that could lug down or overspeed an engine drivingly coupled to the first and second pumps. These engine speed deviations could reduce the overall efficiency of a work machine incorporating the hydraulic system of the '436 patent.
The disclosed control system is directed to overcoming one or more of the problems set forth above.